Seagrass-Mediated Phosphorus and Iron Solubilization in Tropical Sediments.

Tropical seagrasses are nutrient-limited owing to the strong phosphorus fixation capacity of carbonate-rich sediments, yet they form densely vegetated, multispecies meadows in oligotrophic tropical waters. Using a novel combination of high-resolution, two-dimensional chemical imaging of O2, pH, iron, sulfide, calcium, and phosphorus, we found that tropical seagrasses are able to mobilize the essential nutrients iron and phosphorus in their rhizosphere via multiple biogeochemical pathways. We show that tropical seagrasses mobilize phosphorus and iron within their rhizosphere via plant-induced local acidification, leading to dissolution of carbonates and release of phosphate, and via local stimulation of microbial sulfide production, causing reduction of insoluble Fe(III) oxyhydroxides to dissolved Fe(II) with concomitant phosphate release into the rhizosphere porewater. These nutrient mobilization mechanisms have a direct link to seagrass-derived radial O2 loss and secretion of dissolved organic carbon from the below-ground tissue into the rhizosphere. Our demonstration of seagrass-derived rhizospheric phosphorus and iron mobilization explains why seagrasses are widely distributed in oligotrophic tropical waters.

Introduction

Seagrasses are found in coastal waters worldwide except Antarctica, and seagrass meadows are important high-value ecosystems[1,2] providing numerous ecosystem services in terms of high biodiversity,[3] enhanced sediment carbon sequestration,[4] and coastal zone protection against erosion.[5] In tropical sedimentary environments, strong fixation of phosphorus in the prevailing carbonate-rich sediments,[6−9] and adsorption of phosphorus to insoluble iron(Fe)(III) oxyhydroxides[10] lead to strong nutrient limitation. Phosphorus deficiency can negatively affect several plant processes such as energy transfer, photosynthesis, respiration, enzyme regulation, and the synthesis of nucleic acids and membranes.[11] Phosphorus is mainly absorbed through the rhizome and roots of seagrasses,[12] and despite very low (nM) free phosphorus levels in pore waters of carbonate-rich sediments in oligotrophic waters, seagrasses thrive and sustain high primary production forming extensive meadows in such tropical habitats.[13] Seagrasses growing in carbonate-rich sediments also suffer from Fe deficiency[14] with potentially adverse effects on photopigment concentrations in leaves. Tropical seagrass species, such as Cymodocea sp., Halodule sp., Halophila sp., and Thalassia sp., thus seem to have evolved phosphorus, and possibly iron, acquisition mechanisms to support their nutrient requirements, but experimental demonstration of plant-mediated nutrient mobilization from sediments is hitherto lacking.[15]

It has been suggested that seagrass-generated rhizospheric phosphorus solubilization is linked to (i) the release of organic acids from roots (or organic acids derived by fermentation of root/rhizome exudates, such as sucrose)[16] and (ii) plant-derived decreases of the rhizosphere pH as a result of root/rhizome-released acidic substances and/or radial O2 loss (ROL), which leads to acidification through the oxidation of sediment-produced sulfide.[17,18] Such rhizosphere acidification may result in carbonate dissolution and concomitant phosphate release into the porewater. Seagrasses mainly leak O2 from the root/shoot junctions and the root apical meristem area close to the root-tips,[17,19,20] whereas mature parts of the roots possess barriers to ROL.[21] Barriers to ROL likely improve the efficiency of long-distance gas transport within the aerenchyma and thereby improve the internal plant aeration.[22] The rate of ROL from the below-ground tissue is thus largely determined by the internal O2 concentration gradient, where the leaves functions as O2 source and the below-ground tissue as O2 sink.[23] ROL from the below-ground tissue is higher during active leaf photosynthesis as compared to during night-time, where seagrasses depend on the O2 availability in the ambient water.[17,19,24]

Seagrasses also secrete dissolved organic carbon (DOC) into their rhizosphere as root/rhizome exudates,[25] which fuel the rhizospheric microbial community[26] and have been shown to stimulate N2 fixation in the seagrass rhizosphere via sulfate-reducing diazotrophs, thus relieving nitrogen limitation.[27] Sulfate-reducing bacteria (SRB) are highly abundant in seagrass-inhabited sediments,[28,29] where sulfate reduction rates have been shown to increase during photosynthesis.[30,31] SRB form hydrogen sulfide (H2S), which is toxic to seagrasses.[32] H2S intrusion into the below-ground aerenchymal tissue has previously been reported, especially during unfavorable environmental conditions such as water-column hypoxia, high water-column turbidity, and sedimentation onto seagrass leaves impeding the internal plant aeration.[33,34] The ability of seagrasses to aerate the rhizosphere and alter the rhizospheric biogeochemical processes and geochemical conditions is thus critical for their survival. SRB may play multiple roles in tropical sediments. It has been proposed that high rhizospheric H2S concentrations can reduce insoluble Fe(III) oxyhydroxides to dissolved Fe(II), co-dissolving Fe(III) oxyhydroxide-bound phosphate,[10] in addition to the positive effect that N2-fixing SRB appear to have on the porewater nitrogen availability. In this way, SRB may have both beneficial and detrimental effects on seagrass health. Moreover, root/rhizome exudates may also directly stimulate microbiological Fe(III) reduction, thus further supporting the seagrasses’ Fe demand. However, the mentioned plant-mediated phosphorus and iron mobilization mechanisms remain speculative and have never been demonstrated directly in the seagrass rhizosphere.

Using a novel combination of high-resolution in situ chemical imaging methods (see detailed description in the Supporting Information, notes S1 and Figure S1–S6), we now present the first experimental demonstration of seagrass-derived rhizospheric phosphorus and iron mobilization, thus resolving how tropical seagrasses thrive in nutrient-poor carbonate sediments. The underlying hypotheses were (i) that seagrasses can actively stimulate sulfide production in the rhizosphere leading to reduction of insoluble Fe(III) oxyhydroxides to Fe(II), and (ii) that local acidification of the rhizosphere via ROL (potentially supported by Fe(II) oxidation) results in concomitant dissolution of carbonates and thus releases sediment-bound phosphorus to the porewater. Moreover, diel cycles of ROL may favor microbiological Fe(III) and sulfate reduction in the night, and chemical and microbiological Fe(II) and sulfide oxidation causing rhizospheric acidification during the day; thereby, representing a shift in P sources over a diurnal cycle.

Materials and Methods

Experimental Setup and Study Site

Seagrass specimens of Cymodocea serrulata (R.Br.) Asch. & Magnus and carbonate-rich, tropical marine sediment were collected at Green Island, Cairns, Australia (16° 45′ 26.244″ S; 145° 58′ 25.7376″ E). Prior to experiments, the sediment was sieved to obtain the <1 mm grain size fraction excluding any larger in-fauna, while maintaining essential nutrients, buffering salts and microbes. Seagrass specimens were gently uprooted and transplanted into the sieved sediment within narrow experimental chambers submerged in 20 L aquariums (temperature of 28 °C, salinity = 30) (Figure S1). Halogen lamps (Philips Incandescent 230 V PAR38 80W) illuminated the leaf canopy with a photon irradiance (PAR, 400–700 nm) of ∼500 μmol photons m–2 s–1 (resembling natural mid-day conditions at the study site), as measured with a spherical quantum sensor (US-SQD/L, Walz GmbH, Germany) connected to a calibrated quantum irradiance meter (ULM-500, Walz GmbH, Germany). Water movement and aeration were provided by submerged water- and air-pumps, respectively.

Diffusive gradients in thin films (DGT) gels for imaging of sulfide and phosphate distributions were positioned at the back-wall of the experimental chambers and were separated from the sediment/below-ground seagrass biomass by a fine mesh (plankton mesh DIN 100–60, mesh size 60 μm, thickness of 50 μm), to allow for gel deployment and sampling without disturbing the sediment. Multi-ion gels were covered with a Whatman Nucleopore membrane (pore size 0.2 μm, thickness 10 μm) and positioned in direct contact with the sediment. Planar optical sensor foils for pH and O2 imaging were fixed onto the transparent aquarium wall (Notes S1 and Figure S1).

Seagrass specimens were positioned in the experimental chamber ensuring good contact between the below-ground biomass and the optodes or the DGT gels on the opposite side of the investigated roots, before addition of sieved natural marine sediment from the sampling site (Notes S1).

In situ measurements of the natural dynamics and concentrations of sulfide within the investigated seagrass meadow were obtained by deploying sulfide-sensitive gels mounted in commercial DGT samplers (DGT Research Ltd., www.dgtresearch.com) in the sediment over diurnal cycles (Figure S2). For day-time sulfide measurements, the gel probes were deployed in the seagrass meadow at sunrise and retrieved at sunset, and vice versa for the night-time sulfide measurements, resulting in a DGT exposure period of ∼12 h for both day- and night-time deployments.

Ratiometric Imaging of O2 and pH Distribution in the Seagrass Rhizosphere

We used planar optodes for imaging the distribution of O2 and pH around the below-ground seagrass biomass. A detailed description of O2 and pH planar optode preparation and calibration is given in the Supporting Information (Notes S1). A ratiometric RGB camera setup was used for O2 imaging[35] with a SLR camera (EOS 1000D, Canon, Japan) equipped with a macro objective lens (Macro 100 f2,8 D, Tokina, Japan), a 530 nm long-pass filter (Uqgoptics.com), and an additional plastic filter (#10 medium yellow; leefilters.com) positioned in front of the long-pass filter to reduce the background fluorescence. A 455 nm multichip LED (LedEngin Inc., RS Components Ltd., Corby, UK) combined with a bandpass filter was used for excitation of the O2 planar optode. The LED was powered by a USB-controlled LED driver unit (imaging.fish-n-chips.de). Image acquisition and control of the SLR and LED were done with the software look@RGB (imaging.fish-n-chips.de).

A similar ratiometric approach was chosen for pH imaging[36] using a 2-CCD multispectral camera (JAI AD-080 GE; jai.com) equipped with a video objective lens (1.4/23 CCTV-LENS 400–1000 nm; schneiderkreuznach.com) mounted with a 460 nm long-pass filter (schneiderkreuznach.com) and an additional plastic filter (#10 medium yellow; leefilters.com). The pH planar optode was excited with a high-power 405 nm LED (LedEngin; rs-online.com) with a custom-built LED trigger (National Instruments USB 6008). Image acquisition and triggering of the LED were done via custom-made software (bioras.com).

Acquired RGB color images were split into the red, green, and blue channels and analyzed using the software ImageJ (rsbweb.nih.gov/ij/) (Notes S1 and Figure S3). For O2 concentration images, the red channel (O2 sensitive emission of the indicator dye) and green channel (emission of the inert reference dye) of the color images were divided using the ImageJ plugin Ratio Plus (ratio = red/green). Afterwards, the obtained ratio image was fitted to a previously obtained calibration curve (described in Figure S4) using the Curve Fitting tool of ImageJ (exponential decay function). For pH images, the red channel (pH sensitive emission of the indicator dye) and the blue channel (emission of the reference dye) of the images were divided using the ImageJ plugin Ratio Plus (ratio = red/blue). Subsequently, the obtained ratio image was fitted with a previously obtained calibration curve (described in Figure S4) using a linear fit within the boundaries (pKa ± 1 pH units). Calibrated O2 concentration and pH images were further analyzed in ImageJ.

Gel-Based Rhizospheric Sulfide, Phosphate, Fe(II), and Ca2+ Determination

Here, only a brief summary on gel-based imaging and calibration procedures is given (see further details in the Notes S1).

Single-Element Mapping of Sulfide and Phosphate

Analyte-sensitive gels (precipitated Zr-oxide gels for phosphate and AgI gels for sulfide; calibration curves are shown in Figure S5 and S6) were deployed into the experimental microcosms for 12 h. After gel retrieval, the phosphate-collecting gels were stained with a molybdate blue reagent, and subjected to computer imaging densitometric (CID) analysis.[48] No staining step was necessary for the sulfide bound to the AgI gels, as the formed Ag2S was black, while the AgI background was pale white.[37] The CID of the retrieved gels was done with a flatbed scanner (Canon MG2460, Canon, Japan). The retrieved sulfide and phosphate sensitive gels were fixed flat between two transparent PET sheets to avoid direct contact with the scanner and were scanned at a resolution of 600 dpi. Acquired images were further processed using ImageJ and Origin Pro (OriginLab Corp., USA).

Simultaneous Imaging of Phosphate, Fe(II) and Ca2+

Zr-oxide-SPR-IDA gels for simultaneous sampling of phosphate, Fe(II) and Ca2+[38] were deployed into the experimental mesocosms for 24 h. After deployment, the retrieved Zr-oxide-SPR-IDA gels were dried, mounted onto glass slides and subjected to LA-ICPMS analysis on a UP 193-FX laser ablation instrument (ESI, NWR division) coupled to a Nexion 350D ICPMS (PerkinElmer). Count rates were recorded for several isotopes including 13C, 31P, 44Ca, and 57Fe, where 13C was used as internal normalization standard. Gels were imaged at horizontal spatial resolutions of 96 μm (Figure , dark; Figure S9, light) and 113 μm (Figure , light; Figure S9, dark), and a vertical spatial resolution of 400 μm. Chemical images were generated and arranged using Microsoft Excel (Microsoft Corp., Redmond, USA), ImageJ, Systat SigmaPlot (Systat Software Inc., San Jose, USA), Adobe Photoshop, and Adobe InDesign (Adobe Corp., San Jose, USA).

Figure 4

Co-distributions of seagrass-mediated rhizospheric phosphorus and Fe(II) solubilization coupled to plant-generated pH microheterogeneity at the root/sediment interface during light (photon irradiance of 500 μmol photons m–2 s–1) and dark conditions in carbonate-rich marine sediment inhabited by the tropical seagrass Cymodocea serrulata. (a) Rhizospheric pH, Fe(II) and phosphorus concentrations within the selected region of interest, as shown on the provided illustration of the below-ground plant tissue structure (c). The dotted lines on the chemical images show the position of the roots. (b) Concentration profiles (P1–4) extracted at the positions indicated by the white arrows along the figure edge, showing cross tissue Fe(II) and phosphorus concentrations during light and dark conditions. All images were color-coded, where the color scales depict the sediment pH, Fe(II) and phosphorus concentrations, respectively. The red arrow on the phosphorus scale bar indicates the method detection limit (MDL) for the applied phosphorus sensitive multi-ion gel (Zr-oxide-SPR-IDA). No such arrow is shown for Fe as the MDL was negligibly small in this case.

Results and Discussion

Our results show that the tropical seagrass Cymodocea rotundata is able to mobilize phosphorus and iron around its roots in oligotrophic, carbonate-rich sediments by protolytic dissolution of Ca-phosphates, as well as, by reductive dissolution of insoluble Fe(III)-oxyhydroxides. Below, we describe in detail the multiple biogeochemical pathways resulting in nutrient solubilization in the seagrass rhizosphere.

In Situ Sulfide Distribution and Dynamics

In situ sulfide measurements in carbonate-rich sediments vegetated by tropical seagrasses (Cymodocea rotundata, Cymodocea serrulata, Halodule uninervis, and Syringodium isoetifolium) revealed a net downward movement of the sediment sulfide front during day-time and a concomitant reduction in sulfide concentration within the upper ∼10 cm of the sediments (Figure a). Enhanced re-oxidation of sediment-produced sulfide during day-time is caused by higher ROL from the below-ground seagrass tissue, which is driven by high O2 partial pressure (pO2) in the photosynthetic leaves enhancing gaseous O2 transport to the rhizosphere.[17,20,39] During night-time, seagrasses are completely dependent on passive O2 influx into the leaves from the surrounding water-column,[17,33] leading to diminished ROL, which caused the sulfide front to migrate toward the water/sediment interface increasing the tissue exposure to phytotoxic H2S (Figure a).

Figure 1

(a) In situ distribution of sulfide as determined with sulfide sensitive Agl DGT gel probes during light (photon irradiance of 500 μmol photons m–2 s–1) and dark conditions in a sediment colonised by the tropical seagrass species Cymodocea rotundata, Cymodocea serrulata, Halodule uninervis, and Syringodium isoetifolium. (b) The width of all deployed DGT gels was 18 mm. Acquired images were color-coded with a color scale depicting the sediment sulfide concentration.

Mapping of Rhizospheric Sulfide Concentrations

Microniches at the below-ground tissue/sediment interface exhibited distinct low-sulfide microzones around the root/shoot junctions of the seagrasses in the uppermost part of the rhizosphere, as well as around some of the deeper-lying root-tips, and these oxidized zones markedly expanded during light exposure of the leaf canopy (Figure ). Other microniches around mature parts of the roots exhibited high sulfide concentrations indicative of local stimulation of sulfate reduction, probably owed to root/rhizome exudation of DOC during photosynthesis[30,31,40] and/or depletion of electron acceptors other than sulfate in the absence of ROL-driven re-oxidation processes (Figure ). Such microheterogeneity in the rhizospheric sulfide concentration and distribution can be explained by a high ROL from the root/shoot junctions and apical root meristems of the investigated tropical seagrass Cymodocea serrulata (Figure S7; showing seagrass-generated rhizospheric O2 distributions during light and dark conditions) leading to local sulfide oxidation, which is similar to ROL in other seagrass species.[17,20,24] Seagrass-derived sulfide oxidation furthermore resulted in rhizospheric proton formation and thus localized acidification of the immediate rhizosphere (Figure ), as previously shown for the temperate seagrass species Zostera muelleri and Zostera marina.[17,18]

Figure 2

Distribution of sulfide concentrations in the rhizosphere of Cymodocea serrulata during light (photon irradiance of 500 μmol photons m–2 s–1) and dark conditions. The sulfide concentrations were determined with Agl DGT gel probes sensitive to sulfide. Images were color-coded with a color scale depicting the sediment sulfide concentration.

Figure 3

(a) Rhizospheric pH and phosphorus distributions in carbonate-rich sediment around the tropical seagrass Cymodocea serrulata during light (photon irradiance of 500 μmol photons m–2 s–1) and dark conditions. The enlarged plot focuses on the basal leaf meristem area, that is, the meristematic region of the rhizome. (b) Rhizospheric pH and phosphate concentrations during light and dark conditions as extracted along the cross-tissue line profiles shown in (a). Images were color-coded with color scales depicting the sediment pH and phosphate concentrations, respectively.

pH Heterogeneity

Planar optode imaging demonstrated distinct rhizospheric pH heterogeneity around below-ground tissues of Cymodocea serrulata. Low-pH microniches (as compared to the bulk sediment) were found at the meristematic region of the rhizome, that is, the basal leaf meristem, and around the roots and older parts of the rhizome, with light exposure of the leaf canopy leading to stronger acidification (Figures a and S8) than during dark periods. Other microniches with high pH were observed around the first internode of the rhizome adjacent to the basal leaf meristem, as well as around the prophyllums, that is, single leaves originating from the root/shoot junctions (Figure a). Rhizospheric pH microheterogeneity was caused by localized ROL-driven sulfide oxidation along with the precipitation of Fe(III) oxyhydroxides both causing acidification, and potentially also by DOC leakage stimulating proton-consuming sulfate reduction in line with previous findings in the temperate seagrass species Zostera marina.[18] Seagrass-derived low pH microniches may have been further supported by secretion of organic acids from the roots,[16] although this can lead to both a direct reduction in rhizosphere pH and an indirect increase in pH via stimulation of SRB.

Phosphorus and Iron Solubilization

The local reduction in rhizosphere pH resulted in pronounced phosphorus solubilization at the basal leaf meristem, caused by the acid dissolution of calcium phosphates such as carbonate-fluorapatite (Figure a), where a seagrass-mediated reduction in rhizosphere pH of ∼0.8 pH units at the basal leaf meristem correlated with a localized ∼20-fold increase in porewater phosphorus availability (Figure b).

Imaging of dissolved Fe(II) and phosphorus distributions in the rhizosphere during light and dark conditions demonstrated combined sulfide- and acidification-induced phosphorus and iron solubilization in the carbonate-rich sediments around seagrass roots, especially around the root tips (Figure ; Figure S9). At the tip of the root originating from the fourth root/shoot junction (Figure c), the seagrass-mediated phosphorus solubilization was dominated by low pH-induced dissolution of calcium phosphates and thus the release of calcium-bound phosphorus into the porewater (Figure a and b; Figure S10, showing calcium phosphate dissolution). Around the root originating from the third root/shoot junction (Figure c), the seagrass-driven phosphorus solubilization was due to combined sulfide- and low pH-induced phosphorus release owed to reduction of insoluble Fe(III) oxyhydroxides, as well as calcium phosphate dissolution (Figure a and b; Figure S10). The phosphorus mobilization around roots caused an up to ∼10-fold increase in the rhizospheric phosphorus availability (Figures a and b and S9) and thus presents a major source for satisfying the plant’s P demand.

Tropical seagrasses roughly require 60–175 μmol P m–2 d–1 (based on estimates reported by Fourqurean et al.[41] and Jensen et al.,[8] and assuming a 1:1 ratio between the above- to below-ground biomass and similar % P assimilation in the above- and below-ground tissues). DGT measurement cannot be directly interpreted as an actual porewater concentration (see detailed discussion in the Supporting Information(42)), however, Udy et al.[43] reported porewater phosphate concentrations ranging from 0.6–1.7 μM in sediments around Green Island, which is similar to concentrations reported from seagrass meadows in Florida Bay, USA (0.5 to 3 μM[41]) and the Bahamas (1.5 μM[44]). At Green Island, it would make sense to use the lower end of the phosphate concentration range reported (i.e., 0.6 μM), as these measurements of sediment nutrients were taken less than a decade after tertiary treatment of wastewater from the resort at Green Island was installed.[43] Assuming a porewater phosphate concentration of 0.6 μM or 60 μmol m–2 (calculated from a rhizosphere depth of 10 cm) in the sediments around Green Island, the seagrass-mediated nutrient mobilization mechanisms would enhance the rhizosphere phosphate concentration to ∼600–1200 μmol m–2. The actual porewater phosphate concentration at Green Island (i.e. 60 μmol m−2) would thus be consumed within 1 day, if not renewed or mobilized. When taking the seagrass-mediated phosphorus nutrient mobilization mechanisms into account, the increased porewater phosphate concentration alone would support seagrass growth for up to 10–20 days. However, P pools adsorbed to carbonate sediments are much larger, estimated to ∼20 mmol m–2 by Jensen et al.,[8] which would support tropical seagrass growth for up to ∼1 year, if not renewed.

We also found a pronounced increase in dissolved Fe(II) around the roots of Cymodocea serrulata. This Fe(II) solubilization was caused by reductive dissolution of Fe(III) oxyhydroxides and resulted in a ∼10-fold increase in the rhizospheric Fe(II) concentration (Figure a,b; Figure S9). Such enhanced Fe(II) availability around roots (Figure a,b) could help alleviate potential Fe deficiency in leaves.[14] The slightly lower phosphate concentration around roots in light as compared to darkness (Figure ) was probably due to a higher phosphate absorption rate of seagrass rhizome and roots in the light.[45] In general, we showed that the nutrient mobilization capacity of the tropical seagrass Cymodocea serrulata increased in the light owing to enhanced ROL and potentially also due to root/rhizome exudation of DOC and organic acids (Figure ; Figure S7 and S9).

The high phosphorus and iron mobilization at plant/sediment interfaces as driven by tropical seagrasses colonizing carbonate-rich sediments could potentially be further supported by Fe(III)-reducing and/or phosphorus-solubilizing bacteria if present in the surrounding sediment (e.g., refs (10 and 46)). Direct microbial Fe(III) reduction would lead to further formation of dissolved Fe(II) and phosphate in the adjacent sediment, precipitating sulfide diffusing from anoxic, sulphidic sediment zones as FeS,[47] and thus functioning as an additional protection mechanism against H2S accumulation and intrusion in the rhizosphere. However, direct microbial reduction of Fe(III) would preferably occur in the anoxic microniches of the seagrass rhizosphere with low H2S production[47] and not at the root/shot junctions and the root-tips that are generally considered oxic microenvironments.[17,19,20,24] Nevertheless, this potential additional microbe-driven source of nutrients for tropical seagrasses deserves further attention in future studies.

Tropical seagrasses can thus solubilize phosphorus and iron around their below-ground biomass in carbonate-rich sediments via multiple biogeochemical pathways (Figure ). Seagrass-derived rhizospheric phosphorus mobilization was mainly the result of a low pH-induced dissolution of sediment calcium phosphates combined with a sulfide-induced reduction of Fe(III) oxyhydroxides, as well as direct microbial Fe(III) reduction, leading to local enhancement of dissolved Fe(II) and release of Fe(III)-bound phosphate. Moreover, our results indicate that diurnal cycles in rates of ROL resulted in relative changes in the phosphate and Fe(II) sources to the seagrass, where microbial Fe(III) and sulfate reduction leading to reductive dissolution of Fe(III)oxyhydroxides and thus phosphate and Fe(II) co-solubilization dominated in darkness. Chemical and microbial Fe(II) and sulfide oxidation resulting in rhizospheric acidification and protolytic dissolution of Ca-phosphates, that is, solubilization of phosphate, seemed to be dominating in the light. Phosphorus and iron are key limiting nutrients in tropical environments, including tropical seagrass meadows, owing to their high insolubility in the sediment. Our experimental demonstration of seagrass-driven phosphorus and iron mobilization mechanisms in carbonate-rich sediments now explains how seagrasses can thrive in oligotrophic, tropical waters.

Figure 5

Conceptual diagram of seagrass-derived rhizospheric phosphorus and iron mobilization mechanisms in carbonate-rich sediments. Showing (i) protolytic dissolution of Ca-phosphates, (ii) reductive dissolution of insoluble Fe(III)-oxyhydroxides, and (iii) ligand-promoted dissolution and competitive desorption induced by exuded organic anions.